The quadricyclane, being an important high-energy aerospace fuel, has a density of up to 0.98 g·cm−3, and the ice point lower than −40° C., and the substance can be used in propelling systems of aerospace vehicles, for example, such as rockets, guided missiles, satellites, and space crafts. As compared with kerosene fuels, the quadricyclane has the high-energy advantage, and as compared with hydrazine fuels, it has the advantage of the high safety.
The quadricyclane may be synthesized via a photochemical process, that is, norbornadiene, as the starting material, is used to form the quadricyclane via the intermolecular adduct reaction of norbornadiene with ultraviolet radiations and in the presence of an organic photo-sensitizer or a solid photocatalyst as the catalyst for the reaction. The catalyst is an essential factor for determining the reaction velocity.
The photocatalytic isomerization of norbornadiene to synthesize the quadricyclane uses two kinds of catalysts, one kind being an organic photo-sensitizer, and another kind being a solid photocatalyst.
The organic photo-sensitizer may be dissolved in the reaction solution, and thus it can also be called as a homogenous phase photo-sensitizer. Commonly-used organic photo-sensitizers are organic ketone-based photo-sensitizers, e.g., benzophenone, acetophenone, Michler's Ketone, tetraethyl Michler's Ketone, and diethyl Michler's Ketone.
In the U.S. Pat. No. 5,076,813, a solution containing 110 g norbornadiene and 0.1 g Michler's Ketone is irradiated with a 150 W high-pressure mercury lamp. After the reaction is effected for 37 hours, the resultant conversion rate is 99%, and the selectivity of the quadricyclane is 99%.
In patent publication US2004/0054244 A1, solutions containing 5 ml norbornadiene and 0.32% Michler's Ketone or 0.75% ethyl Michler's Ketone are respectively irradiated with a 400 W high-pressure mercury lamp for 16 hours, the resultant conversion rates are 53.7% and 65.8% respectively.
The solid photocatalyst is not dissolved in the reaction solution, and thus it is also called as a non-homogenous phase photocatalyst. The applicant of the present patent early reports a V-Ti-MCM-41 solid photocatalyst (Applied Catalysis B (2010) 439-445), and a solution containing 5 ml norbornadiene, 0.1 g the catalyst and 500 ml xylene is irradiated with a 400 W high-pressure mercury lamp for 4 hours, the resultant conversion rate being 90%.
However, existing catalysts have various disadvantages. Because the organic photo-sensitizer can be dissolved in reaction solution, its separation and recovery from reaction product and reactants which are not sufficiently reacted will become a problem. Furthermore, the organic photo-sensitizer has a low activity. Thus, in the case that no solvent is added, the reaction time should be 20 hours or above, and in the case that a dilution solvent is added, the reaction time may be decreased to less 10 hours. However, because of the dilution action of the solvent, the treating amount of reactants per unit time is low, and thus the need of a large-scale preparation is hard to meet.
Although the use of a solid photocatalyst can solve the problem of separation and recovery of the catalyst from the reaction system, the reaction should be carried out in the presence of large quantities of solvents. Thus, the treating amount of reactants in unit time is limit, thereby restricting the application of the solid photocatalyst.
Hence, a novel catalyst having a higher activity for the photo-catalytic isomerisation of norbornadiene to prepare quadricyclane is desired. The catalyst is desired, in the case that no solvent is added, to realize a quadricyclane yield of higher than 90% within 10 hour reaction time, and the catalyst should have a stable activity and be easily separated and recovered.
The present invention is aimed to solve the above problems.